A. Field of the Invention
This invention relates to systems and components of systems used in certain chemical testing apparatus, in particular to a system for control of back-pressure in chemical analysis systems which involve small volume instrumentation.
B. Prior Art
While there have always been systems which involve the maintenance of pressure, especially back pressure, within prescribed limits, they generally have involved systems in which there was a large fluid flow controlled by manually or electrically operated pressure valves. In such systems, there was usually not a requirement for stringent control of gas pressure to tight tolerances.
There are, however, certain applications which demand extremely rigid governance of pressure. Among them are certain chemical analytic or laboratory testing systems in which there is a very small flow of gas through a small volume chamber such as a microreactor or micro-pilot plant system. An example of the latter is the Series 8000 micro-pilot plant made and marketed by Chemical Data Systems, Inc. of Oxford, Pennsylvania. Into the chamber a substance under test, such as a catalyst, is inserted and then various types of gases are fed through the chamber. To accurately test the properties of the catalyst as it affects the gases fed to the chamber, it is important that the pressure and/or temperature within its enclosure be tightly controlled. For rapid and accurate temperature control, a micro-reactor tube subassembly such as the one disclosed in the copending application of Bowe et al, U.S. Ser. No. 463,339 filed Feb. 2, 1983 entitled "Chemical Microreactor Having Close Temperature Control" may be employed. The problem of attainment of rigid pressure (back-pressure) controls for use downstream of the chamber in such miniature or micro-flow systems also presented problems. It was found that conventional pressure control valves for larger scale apparatus which had motorized drives did not have sufficient rapidity of response to maintain the pressure within prescribed bounds. It was then decided to try to use ordinary, larger scale, metering (rather than pressure) valves and operate them with some sort of motorized drive, but this also was found unsatisfactory from a number of standpoints including their sluggishness.
Resort then was had to a commercially available flow control valve intended for relatively large flow rates upstream of the chamber. One such valve had the advantage of relatively high frequency response, but its inlet and outlet orifices were much larger than needed for the small flow involved. Sometimes actuation of this valve would cause such a large pressure drop that the stability of the pressure was reduced. Moreover, the valve could be set into oscillation under certain circumstances. Another disadvantage of those valves is the fact that, being located upstream of a reactor, they were not designed to operate on gases raised to high temperatures. Thus, if the gases involved had molecular weights which were considerably higher than oxygen or hydrogen, for example, upon passing out of the high temperature chamber, they would condense on the inner walls of the tubing leading from the chamber outlet. Finally, that commercially available flow control valve had a valve element that was not sufficiently resistant to certain types of chemicals or to the elevated temperatures involved.
In accordance with the present invention, therefore, it is among the objects of the present invention to provide a back-pressure valve assembly for such small-volume applications that has sufficiently fast frequency response, greater resistance to chemical attack and high temperature damage, less susceptibility to oscillation, and has means for preventing condensation of certain gases on the inner walls of tubing coupled to it.
Once the appropriate pressure valve had been constructed, it was found that automatic control of the driving means (solenoid) of such a valve presented problems. It was soon realized that driving the solenoid with a signal from a servo amplifier of Type 1 construction was incapable of maintaining the pressure of the chamber within the very tight bounds demanded by this precision system. A Type 2 servo system which integrates out the error so as to reduce the difference between the actual and set-point voltage was designed and found better for this purpose.
It is therefore also among the objects of the present invention to provide a Type 2 servo system which can cooperate with a back-pressure valve of the type described, to maintain a gas pressure within the chamber that is within one pound of the set-point pressure.